Characteristics are being studied for the binding of bovine neurophysins to the neurohypophyseal peptide hormones, oxytocin and vasopressin. Direct binding analyses are being carried out by equilibrium dialysis, using C14-labelled peptide ligands. The possibility of studying binding on insoluble affinity matrices also is being explored. The relationship between the aggregation properties of the neurophysins and the hormone-neurophysins interaction is being investigated. In this respect, ultracentrifuge studies of the neurophysins in the presence and absence of hormone or peptide analogues are being carried out using sedimentation-equilibrium methods. In addition, possible biosynthetic interrelationships between the neurophysins and the neurohypophyseal peptide hormones are being investigated. To date, efforts in this direction have been limited to studies of the stability of the neurophysins with respect to susceptibility to disulfide interchange, alone or with peptide ligands present. The results of studies so far indicate a non-ideal saturation of neurophysin by peptide ligands which takes the form of positive cooperativity. There is evidence for a causative relationship between this property and alteration of neurophysin aggregation by peptide ligands. The possibility that neurophysins are synthesized within a larger precursor form is consistent with our results in disulfide$ interchange studies.